High-Efficiency Air Source Heat Pump Installation in a Grade II Listed Buckinghamshire Property

Welcome to the BetaTeach newsletter. We share case studies from engineers in the Guild of Master Heat Engineers to help consumers find top installers, inspire gas and oil engineers to expand their skills, and provide reliable knowledge for third-sector professionals.

Overview

A Viessmann Vitocal 150-A air source heat pump was installed in a large, three-storey, timber-framed Grade II listed building in Buckinghamshire. The house, which spans three floors, each with five rooms across and one room deep, presented unique challenges due to its size and heritage status. The project's success challenged conventional industry beliefs regarding the suitability of heat pumps for older, less insulated properties, achieving a Seasonal Coefficient of Performance (COP) of 4.8. This case study details the innovative design and installation techniques that made this outcome possible.

Master Heat Engineers

Key Specifications

• Heat Pump 16 kW Viessmann Vitocal 150-A

• Estimated Calculated Heat Loss 12.3kW

• Hot Water Cylinder Newark HG Series with 6m² coil

• Heating Distribution Underfloor heating (1,800 sq ft) on the ground floor; column radiators on upper floors

• Pipework Multilayer composite (MLCP), copper, and insulated underground piping

• System Design Open-loop, bufferless system

• Power Source Photovoltaic (PV) panels with battery storage

The Challenge: Overcoming Conventional Wisdom

The owner, Alex Hilton, a former Head of Sustainability at HMRC, sought to replace his home's 1980s gas boiler. Several heating engineers advised against a heat pump, citing the building's age and perceived inefficiency. Hilton's conviction, informed by his professional background, was that a building's performance is not a barrier to a heat pump, but rather a reflection of its overall heat loss, a problem that can be addressed. The existing setup, consisting of a gas boiler with a network of microbore piping, was inefficient and poorly suited for a like-for-like replacement due to the property's listed status.

The project required a contractor who shared this perspective. Custom Renewables were chosen for their expertise in low-temperature system design. A detailed heat loss survey confirmed that the property’s heat loss was approximately 12.3 kW, making it a viable candidate for a 16 kW heat pump.

Design and Installation Innovations

The installation faced unique constraints due to the property's Grade II listed status. These included restrictions on drilling external walls and a requirement for traditionally styled column radiators.

System Design

The system was designed as an open-loop configuration without a buffer tank, a departure from standard practice. Liam Barry from Custom Renewables re-calculation of the Viessmann Vitocal 150-A’s internal pump head (7.5m) determined that a buffer tank was unnecessary, reducing system complexity and cost.

Pipework and Distribution

The old microbore piping was replaced. The ground floor received underfloor heating, which operates at a lower flow temperature, improving system efficiency. The upper floors were fitted with radiators. The pipework was discreetly run through existing eaves and cupboards to minimise visual impact, a key consideration in a heritage building. An innovative 5-meter underground trench was used for the pipe connection to the outdoor unit, insulating the fluid and protecting the line.

Efficiency Upgrades

The property’s thermal envelope was improved with additional attic insulation and secondary glazing, ensuring the heat pump could operate effectively.

Performance and Results

Since its commissioning, the system has consistently performed at a high level. Open Energy Monitor data shows the installation maintaining a Seasonal COP of 4.8, placing it at or near the top of the performance charts.

The system's low-temperature output provides a consistent, comfortable warmth throughout the house, unlike the high-temperature cycles of the previous gas boiler. The family has reported a noticeable increase in comfort and a significant reduction in drafts. The integration with PV panels and battery storage has resulted in minimal reliance on grid electricity. This case study demonstrates that with informed design and a commitment to detail, high-efficiency heat pump systems can be successfully deployed in historic buildings, delivering outstanding performance and comfort.

Estimated Heat Loss (kW) at Various Outside Temperatures

If you want to know more about how heat works, I’ll be posting a deep-dive on What is heat anyway? 

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Podcast

Take a listen to one of this weeks listed Master heat engineers Barry Sharp from Renewable Heat. Barry chats to his heat pump customer Paul Eastwood from OFGEM (formally at the Foreign Office).

Closing Thoughts

It's been a real pleasure to launch the newsletter with this first case study from Liam and Mark at Custom Renewables. Liam epitomises what I've been helping people to understand in the industry: that the competence of an engineer usually comes down to their commitment to taking ownership of their own learning. It's not just about going on courses. It's what you do with the information you've received via a course. Liam is well known among the heating industry for going back home after doing courses and then practicing. This could mean making mock-up scenarios of heating systems and then calculating the heating load required as well as flow rates and temperatures. It's like anything else in life e.g. no one becomes a great guitarist or golf player by going to lessons. They become great by practicing what they learn. Over the course of the next few months I'll be discussing training within this sector.

This install also considered the water quality inside the heating system and used VDI 2035 methods promoted by my friend Ricky at Thoroughflush. Again, I'll be discussing this method over the next few months as it is a topic which highlights where marketing has gone wrong in the UK heating sector and how manufacturers have not understood how to properly interpret the building regulations. But that's for another time.

Take care and please share... all the best

Nathan

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